Carbon Nanotube Thin Film Transistors for Flexible Electronics

Abstract

The main objective of the emerging field of flexible macroelectronics is to develop scalable and cost effective routes to incorporate active circuit elements such as thin film transistors (TFTs) onto large-area flexible substrates. This dissertation reveals the promising potential of single-walled carbon nanotube (CNT) networks, synthesized using a floating catalyst (aerosol) chemical vapor deposition method, as an active layer material for TFTs. Direct dry or transfer printing techniques were developed for the deposition of as-grown CNTs in the form of random networks at room temperature and under atmospheric pressure onto any type of substrate, including heat-sensitive flexible materials. Patterned assembly of nanotubes and a lithography-free device fabrication technique were demonstrated. The results presented in this dissertation propose an alternative solution to remove existing manufacturing bottlenecks by capitalizing on the superb properties of pristine CNTs and minimizing the intermediate steps in the process flow between the nanotube synthesis and application, with the purpose of lowering manufacturing costs.

The CNT growth conditions were optimized to produce high-quality, long nanotubes (up to 10 µm) with a narrow diameter distribution (mean diameter around 1.1 nm). The nanotube network material was integrated directly from the gas-phase synthesis reactor into TFTs with the channel lengths up to 100 µm. The effect of network morphologies, obtained by four different aerosol-based nanotube deposition techniques, on device performance was studied. The results demonstrated that long, partially aligned nanotubes with larger junction area and controlled density exhibit superior characteristics. Optimized CNT TFTs showed simultaneously high on/off current ratio of 6×106 and carrier mobility of 35 cm2V-1s-1 based on a parallel plate model for the gate capacitance estimation. When evaluated by a more rigorous analytical model, the highest mobility reached 1236 cm2V-1s-1 with concurrent on/off ratio of 1.5×104, making these TFTs attractive for post-silicon technologies.

Successful operation of CNT TFTs and functional integrated circuits on flexible and transparent plastic substrates was shown. Future scalability of the fabrication process, for example, by using high-throughput printing techniques, opens new routes toward the realization of large-area flexible electronics.